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2.7 Flashcards

1
Q

DNA replication is a…

A

semi-conservative process

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2
Q

when a new double-stranded DNA molecule is formed:

A
  • One strand will be from the original template molecule

- One strand will be newly synthesised

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3
Q

DNA replication occurs because

A

each nitrogenous base can only pair with its complementary partner (CG) (AT)

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4
Q

Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase:

A
  • Each new strand formed will be identical to the original strand separated from the template
  • The two semi-conservative molecules formed will have an identical base sequence to the original molecule
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5
Q

The theory that DNA replication was semi-conservative was confirmed by which experiment?

A

Meselson-Stahl experiment in 1958

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6
Q

Prior to this experiment, three hypotheses had been proposed for the method of replication of DNA:

A
  • Conservative Model
  • Semi-Conservative Model
  • Dispersive Model
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7
Q

Conservative Model

A

An entirely new molecule is synthesised from a DNA template (which remains unaltered)

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8
Q

Semi-Conservative Model

A

Each new molecule consists of one newly synthesised strand and one template strand

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9
Q

Dispersive Model

A

New molecules are made of segments of new and old DNA

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10
Q

Meselson and Stahl were able to experimentally test the validity of these three models using…

A

radioactive isotopes of nitrogen

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11
Q

Meselson and Stahl procedure with nitrogen

A

DNA molecules were prepared using the heavier 15N and then induced to replicate in the presence of the lighter 14N and DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules

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12
Q

The results after two divisions supported the semi-conservative model of DNA replication

A
  • After one division, DNA molecules were found to contain a mix of 15N and 14N, disproving the conservative model
  • After two divisions, some molecules of DNA were found to consist solely of 14N, disproving the dispersive model
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13
Q

The process of DNA replication is coordinated by two key enzymes

A

helicase and DNA polymerase

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14
Q

Helicase

A
  • Helicase unwinds the double helix and separates the two polynucleotide strands
  • It does this by breaking the hydrogen bonds that exist between complementary base pairs
  • The two separated polynucleotide strands will act as templates for the synthesis of new complementary strands
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15
Q

DNA Polymerase

A
  • DNA polymerase synthesises new strands from the two parental template strands
  • Free deoxynucleoside triphosphates (nucleotides with 3 phosphate groups) align opposite their complementary base partner
  • DNA polymerase cleaves the two excess phosphates and uses the energy released to link the nucleotide to the new strand
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16
Q

The polymerase chain reaction (PCR)

A

is an artificial method of replicating DNA under laboratory conditions

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17
Q

PCR is used to

A

amplify large quantities of a specific sequence of DNA from an initial minute sample. Each reaction doubles the amount of DNA – a standard PCR sequence of 30 cycles creates over 1 billion copies (230)

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18
Q

PCR occurs in a thermal cycler and uses variations in temperature to control the replication process via three steps:

A
  1. Denaturation – DNA sample is heated (~90ºC) to separate the two strands
  2. Annealing – Sample is cooled (~55ºC) to allow primers to anneal (primers designate sequence to be copied)
  3. Elongation – Sample is heated to the optimal temperature for a heat-tolerant polymerase (Taq) to function (~75ºC)
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19
Q

Taq polymerase is an enzyme isolated from

A

the thermophilic bacterium Thermus aquaticus

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20
Q

Thermus aquaticus optimal temperature

A

As this enzyme’s optimal temperature is ~75ºC, it is able to function at the high temperatures used in PCR without denaturing

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21
Q

Transcription

A

the process by which an RNA sequence is produced from a DNA template

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22
Q

transcription: role of DNA polymerase

A

RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands

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23
Q

transcription: role of ribonucleoside triphosphates

A

When the DNA strands are separated, ribonucleoside triphosphates align opposite their exposed complementary base partner

24
Q

transcription: role of RNA polyermase

A

RNA polymerase removes the additional phosphate groups and uses the energy from this cleavage to covalently join the nucleotide to the growing sequence

25
Q

transcription: final part

A

Once the RNA sequence has been synthesised, RNA polymerase detaches from the DNA molecule and the double helix reforms

26
Q

Gene

A

The sequence of DNA that is transcribed into RNA is called a gene

27
Q

antisense strand

A

The strand that is transcribed is called the antisense strand and is complementary to the RNA sequence

28
Q

sense strand

A

The strand that is not transcribed is called the sense strand and is identical to the RNA sequence (with T instead of U)

29
Q

Transcription of genes occur in the…

A

nucleus (where DNA is), before the RNA moves to the cytoplasm (for translation)

30
Q

The base sequence of an mRNA molecule encodes the…

A

production of a polypeptide

31
Q

The mRNA sequence is read by the ribosome in triplets of bases called…

32
Q

Each codon codes for…

A

one amino acid with a polypeptide chain

33
Q

The order of the codons in an mRNA sequence determines the order of…

A

amino acids in a polypeptide chain

34
Q

The genetic code is the set of rules by which…

A

information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells

35
Q

The genetic code identifies the…

A

corresponding amino acid for each codon combination

36
Q

how many codon possibilities are there?

A

As there are four possible bases in a nucleotide sequence, and three bases per codon, there are 64 codon possibilities (43)

37
Q

The coding region of an mRNA sequence always begins with a…

A

START codon (AUG) and terminates with a STOP codon

38
Q

Typically the genetic code shows the codon combinations expressed on an…

A

mRNA molecule

39
Q

translation is the process of…

A

protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain

40
Q

translation: role of ribosomes (binding)

A

Ribosomes bind to mRNA in the cytoplasm and move along the molecule in a 5’ – 3’ direction until it reaches a start codon (AUG)

41
Q

translation: role of anticodons

A

Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. AUG = UAC)

42
Q

translation: role of each tRNA molecule

A

Each tRNA molecule carries a specific amino acid (according to the genetic code)

43
Q

translation: role of ribosomes (catalyse)

A

Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via condensation reactions)

44
Q

translation: when does stop codon occur

A

The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon

45
Q

what happens when translation ceases?

A

At this point translation ceases and the polypeptide chain is released

46
Q

Translation Mnemonic

A

Mr Cat App

47
Q

Translation Mnemonic meaning

A 
Messenger RNA  (goes to…)
Ribosome  (reads sequence in …)
Codons  (recognised by …)
Anticodons  (found on …)
Transfer RNA  (which carries …)
Amino acids  (which join via …)
Peptide bonds  (to form …)
Polypeptides
48
Q

The genetic code is?

A

universal – almost every living organism uses the same code (there are a few rare and minor exceptions)

49
Q

The ability to transfer genes between species has been utilised to produce…

A

human insulin in bacteria

50
Q

insulin mass production process

A

The gene responsible for insulin production is extracted from a human cell
It is spliced into a plasmid vector (for autonomous replication and expression) before being inserted into a bacterial cell
The transgenic bacteria (typically E. coli) are then selected and cultured in a fermentation tank (to increase bacterial numbers)
The bacteria now produce human insulin, which is harvested, purified and packaged for human use (i.e. by diabetics)

51
Q

insulin mass production name

A

Recombinant Gene Transfer

52
Q

mRNA is a complementary copy of a…

A

DNA segment (gene) and consequently can be used to deduce the gene sequence

53
Q

mRNA → DNA

For converting a sequence from mRNA to the original DNA code, apply the rules of complementary base pairing:

A

Cytosine (C) is replaced with Guanine (G) – and vice versa
Uracil (U) is replaced by Adenine (A)
Adenine (A) is replaced by Thymine (T)

54
Q

mRNA → Polypeptide

In order to translate an mRNA sequence into a polypeptide chain, it is important to establish the correct reading frame

A

The mRNA transcript is organised into triplets of bases called codons, and as such three different reading frames exists

  • An open reading frame starts with AUG and will continue in triplets to a termination codon
  • A blocked reading frame may be frequently interrupted by termination codons

Once the start codon (AUG) has been located and reading frame established, the corresponding amino acid sequence can be deduced using the genetic code

55
Q

open reading frame

A

An open reading frame starts with AUG and will continue in triplets to a termination codon

56
Q

blocked reading frame

A

A blocked reading frame may be frequently interrupted by termination codons

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